1. Field of the Invention
The present invention relates to a carbon fiber woven fabric made of flat carbon fiber yarn, which exhibits excellent characteristics as a fiber composite material, its weaving method and weaving apparatus, and more particularly to a thin carbon fiber woven fabric, which uses flat carbon fiber yarn and which features uniform fiber density, its weaving method and weaving apparatus.
2. Description of Related Arts
The carbon fiber woven fabric, which is made of carbon fibers having high specific Young's modulus and high specific strength, is normally woven by a general shuttle loom or rapier loom. Such carbon fiber woven fabric is frequently used as a reinforcing base fabric for composite materials including carbon fiber reinforced plastic (hereinafter referred to as "CFRP") by compounding it with a matrix resin and molding them into a specific shape.
As a composite material using such a reinforcing base fabric, the CFRP, for example, is starting to be used as a structural material or the like for aircraft owing to its excellent performance. To further expand the application field of the CFRP, it is important to reduce the cost of the molding and also of the carbon fiber and the reinforcing base fabric for carbon fiber woven fabric (hereinafter referred to as "CF fabric").
The carbon fiber yarn (hereinafter referred to as "CF yarn") can be manufactured with higher productivity in the precursor, oxidation process, and carbonization process and at lower cost as the yarn size increases.
A typical CF fabric, however, is made of CF yarn which coheres to have a nearly round cross section; therefore, in a woven state, the cross section of the CF yarn at a point at which the warp and weft cross each other is elliptic, with the weaving yarn being significantly crimped. This trend is conspicuous especially in a CF fabric which uses CF yarn with a largre yarn size because warp and weft of a large yarn size cross each other.
Hence, in the CF fabric with considerably crimped CF yarn, the fiber density tends to be nonuniform, preventing high strength, which is a feature of carbon fiber, from being fully exhibited. In addition, the CF fabric using CF yarn with a large yarn size is normally accompanied by more weight of woven fabric (g/m.sup.2) and increased thickness. This adversely affects the resin infiltration property when manufacturing a preimpregnated material (hereinafter referred to simply as "prepreg"), or molding a fiber reinforced plastic (hereinafter referred to as "FRP").
Therefore, CFRP produced by using a CF fabric woven with CF yarn with a large yarn size inevitably has more voids present in the resin, failing to exhibit high strength.
On the other hand, in the case of a CF fabric which is woven with CF yarn of a large yarn size and which has a smaller weight of woven fabric, the gaps formed between CF yarns are larger. For this reason, forming CFRP using the CF fabric with a smaller weight of woven fabric presented a disadvantage in that the CF yarn content is low and resin voids occur intensively in the gaps which are formed between the CF yarns, thus making it impossible to acquire a high-performance CFRP.
Unexamined Japanese Patent Publication (KOKAI) No. 58-191244 discloses a thin woven fabric, which uses a thin, wide and flat CF yarn, and has a thickness of 0.09 mm or less and a weight of woven fabric of 85 g/m.sup.2 or less, and its weaving method which eliminate the disadvantage described above. Since this thin woven fabric is extremely thin, the crimps of the weaving yarn are small; therefore, high reinforcing effect is ensured, making it a good basic fabric for molding a thin CFRP.
The CF fabric using such a flat CF yarn is woven by successively shedding, by a heald, a warp supplied from a beam wound with the required number of CF yarns or a sheet-like warp supplied from a CF yarn bobbin which is mounted on a creel, and by intermittently inserting weft into the open sheds using a shuttle or rapier.
In this case, the warp is supplied through a beam or directly from a bobbin as described above. In either way, there are two methods; one is the transverse take-out wherein the warp is taken out, while slowly turning the CF yarn bobbin, by pulling it out in a direction so that it crosses with the rotary axis at right angle, and the other is the longitudinal take-out wherein the warp is taken out by pulling it out in a direction of the axis of the bobbin.
Since the warp is paid out in the direction of the axis of the bobbin in the longitudinal take-out, this method is more advantageous than the transverse take-out in that the warp can be paid out instantly at high speed without drag. In the longitudinal take-out, however, the warp is twisted once each time the warp is paid out from the bobbin. Thus, the flatness of the warp at the twisted portion is crushed and partially squeezed. This presents a problem in which a CF fabric with a uniform warp yarn width cannot be obtained.
To solve such a problem, a weaving method can be considered whereby to prevent the warp from being twisted by using the transverse take-out instead. In a conventional heald, however, the mail is made to be longer than it is wide in order to minimize the chance of interference with warp. This causes the mail or the comb, which makes warp density uniform, to crush the flatness of warp, and a fabric with uniform yarn width throughout the fabric cannot be produced.
On the other hand, the weft must be quickly supplied to the above-mentioned open sheds; therefore, the weft supplying speed needs to be higher than that of the warp. Hence, to quickly take out the weft from the fiber yarn bobbin, the longitudinal take-out, whereby the weft is paid out in the direction of the axis of the fiber yarn bobbin, is widely used. This, however, presents a problem in that the yarn is twisted.
To solve such a problem, in Unexamined Japanese Patent Publication No. 2-74645, a method, wherein a bobbin with weft wound around it is actively rotated by a motor and the weft in a length required for inserting it is retained making use of gravity, is suggested.
However, this method wherein the bobbin is actively rotated presents a problem in that the take-out speed must be changed according to the amount of weft wound round the bobbin. In addition, the motor is intermittently run in accordance with the insertion of weft, and therefore, the motor is started and stopped frequently, causing the flat CF yarn to be slackened and thus twisted due especially to the lag in the stopping motion.
Further, to minimize the crimp of weaving yarn at a crossing point of warp and weft, it is desirable that the fiber constituting the weaving yarn has as large a yarn size as possible, the weaving yarn is thinner, and the warp and weft have yarn intervals that are nearly equal to their yarn width in making up the fabric.
On the other hand, however, the yarn width tends to considerably increase as the yarn size of weaving yarn increases, thus the flatness of yarn is crushed at the time of weaving, making it impossible to produce a fabric with a uniform fiber density. There is another problem in that, if weaving yarn is extremely thin and has an extremely small width, then the rigidity in the direction of the yarn width becomes low, causing the flatness of yarn to be easily crushed at the time of weaving.
In this case, it is desirable to apply a sizing agent to the weaving yarn to maintain the flatness of the weaving yarn. Excessive application of the agent, however, will prevent the resin infiltration for CFRP at the time of molding, and the resulting CFRP will fail to exhibit high strength. The desirable amount of the sizing agent to be applied is 0.5 to 2.0 percentage by weight.
Further, in the thin woven fabric and its weaving method disclosed in Unexamined Japanese Patent Publication No. 58-191244 previously mentioned, to form medium or thick CFRP, an enormous number of pieces of base fabric or woven fabric prepreg must be laid up. Thus, this method is disadvantageous in that the formed CFRP costs high and the forming work is extremely time-consuming.
Hence, conventionally, using a CF yarn with a larger yarn size prevents acquisition of a CFRP featuring excellent strength, and no satisfactory method or apparatus is available for weaving a CF fabric from a flat CF yarn. There has been demand for satisfactory method or apparatus for that purpose.